This invention relates generally to shift registers and particularly to redundant shift registers for scanned devices, such as liquid crystal displays (LCD's).
A liquid crystal display having redundant select line scanners (shift registers) is shown in FIG. 1 in simplified form. The display 10 includes an array of liquid crystal elements 11 arranged in rows and columns. Each liquid crystal element is associated with a switching device 12, such as a thin film transistor (TFT). The gate of each TFT 12 is connected to a select line 13 so that a complete row of TFT's is simultaneously biased by each select line 13. Substantially identical select line scanners 21 and 22 are connected to opposite ends of the select lines 13 by output terminals 17 and 18, respectively. The select line scanners provide the biasing voltages to the select lines. The drain of each TFT is connected to a data line 14 which provides a grey scale (brightness) signal to the liquid crystal element 11 associated with a conductive TFT 12. The data lines 14 are individually connected to the output terminals 15 of a data line scanner 16.
Select line scanners 21 and 22 are shift registers, each of which has a plurality of cascaded stages connected to opposite ends of each of the select lines 13. The brightness signals are simultaneously applied to all of the data lines 14 by data line scanner 16 so that all liquid crystal elements 11 associated with the biased select line are simultaneously charged to various voltages in accordance with the brightness signals received from data line scanner 16.
Liquid crystal displays for television and computer monitors have a large number of columns of liquid crystal cells, for example 1,440. A display with this number of columns and having the standard 3.times.4 aspect ratio would have approximately 1,080 select lines. Select line scanners 21 and 22 each would include 1,080 stages, and each stage includes 6 TFT's, for the example shown. Accordingly, there is a relatively high probability that at least one stage of one of the shift registers includes a flaw. The use of redundant registers allows a faulty stage of one register to be replaced by the corresponding stage of the other register. For this reason there is a need for a method of testing the redundant registers. There, therefore is also a need for a technique to electrically isolate the corresponding stages of the redundant registers during testing. The present invention fulfills these needs.